Engines may increase output power by using boosting devices that compress intake air. Since charge compression increases air temperature, charge air coolers may be utilized downstream of a compressor to cool the compressed air, further increasing the potential power output of the engine. As intake air passes through the charge air cooler and is cooled below a dew point, condensation occurs. The condensate may be accumulated at a trap and delivered to the running engine subsequently, at a controlled rate. The introduction of water into the engine, however, can increase the likelihood of misfire events and decrease the likelihood of knock. Engine control systems may have to employ various knock and misfire control approaches to address water and humidity in the intake air.
One example approach for addressing low humidity induced knock is shown by Sasaki et al. in US 2011/0303187. Therein, a knock-limit ignition timing is adjusted based on deviations in a fuel octane content from a basic fuel octane content as well deviations in ambient humidity from a basic ambient humidity. This allows knock and misfire events arising due to a sudden change in fuel octane content and low ambient humidity to be reduced.
However the inventors herein have identified potential issues with such a knock control approach. Even with the adjusted knock-limit ignition timing, potential spark advance opportunities caused by the knock mitigation properties due to condensate ingestion may not be sufficiently addressed. Specifically, condensate formation may involve various factors including, but not limited to, ambient humidity. Other factors that may affect condensate formation at the charge air cooler include, for example, mass air flow, ambient temperature, charge air cooler outlet temperature, ambient temperature, EGR, etc. Thus, there may be conditions ambient humidity is low but condensate formation is high. If spark ignition timing is not adjusted during those conditions, the ingested condensate can slow the burn rate of combustion and the unadjusted ignition timing can degrade combustion efficiency. Likewise, there may be conditions when ambient humidity is high but condensate formation is low. If spark timing is not adjusted during those conditions, combustion efficiency may again be reduced.
In one example, some of the above issues may be addressed by a method for a boosted engine comprising purging condensate from a charge air cooler to an intake manifold and adjusting spark timing based on an amount of condensate purged per cycle. In this way, misfire can be reduced and combustion efficiency maintained when condensate is purged from the cooler to the engine intake.
As one example, an amount of condensate collected at a charge air cooler may be monitored during engine operation. When the condensate level is higher than a threshold, a purging of the condensate to the engine intake may be initiated. Based on an amount of condensate being purged per engine cycle, spark timing may be adjusted. As an example, the condensate may be purged during a tip-in wherein the increased air flow to the engine (to meet the operator torque demand) purges condensate from the charge air cooler into the engine intake. Herein, the purging may occur over a relatively smaller amount of time with a larger amount of condensate purged per engine cycle. Due to the resulting higher intake manifold humidity (due to the higher ingestion of condensate), borderline spark limits may be advanced, and spark timing during the purging may also be advanced towards MBT (or an amount of spark retard may be limited or reduced).
As another example, the condensate may be purged by actively increasing air flow to the engine while maintaining engine torque. Herein, the purging may occur over a relatively longer amount of time with a smaller amount of condensate purged per engine cycle. During the purging, spark timing may be retarded to maintain the torque.
In this way, spark adjustments may be performed while condensate is purged from a charge air cooler to an engine intake to reduce misfire events and driver awareness. By advancing borderline knock limits and spark timing when condensate is purged at a higher rate, the increased intake manifold humidity from the purging can be advantageously used to limit knock. By retarding spark timing and adjusting knock limits when condensate is purged at a lower rate, engine torque can be maintained during the purging. Overall, purging can be accomplished without vehicle performance concerns.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.